1. Field of the Invention
The present invention relates to a reflection type liquid crystal display which can increase the contrast ratio.
Further, the present invention relates to a reflection type liquid crystal display in which angles of a polarization plate, a half wavelength phase difference film, and a quarter wavelength phase difference film are adjusted so that change in retardation of a liquid crystal layer depending on the viewing angle is cancelled by the change in retardation of the phase difference films depending on the viewing angle.
This application is based on Japanese Patent Applications Nos. 10-099144 and 10-346241, the contents of which are incorporated herein by reference.
2. Description of the Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98
In recent years, reflection type liquid crystal displays which do not use a back light, which can reduce consumption of power, and which are used in various mobile devices driven by batteries have become widely used. The most popular system currently in use is a twisted nematic system (hereinafter referred to as TN). In the following, the prior liquid crystal display will be explained with reference to figures.
FIG. 20 is a cross-sectional side view showing the first reflection type liquid crystal display using the TN system of the background art. The first conventional reflection type liquid crystal display 400 comprises a pair of transparent substrates which are an upper substrate 401 and an lower substrate 402 arranged in parallel and separated from each other by approximately 5 xcexcm, and a liquid crystal layer 403 whose orientation is twisted by 90 degrees and which is provided between the substrates 401 and 402.
Polarization plates 406 and 407 are laminated on the surfaces of the transparent substrates which are not in contact with the liquid crystal layer. The lower substrate 402 has a reflector plate 408 on the outer surface of the polarization plate 407. Transparent electrodes 404 made of a transparent conductive material such as indium tin oxide (ITO) are laminated on the surfaces of the upper and lower substrates 401 and 402 in contact with the liquid crystal layer. On these transparent electrodes 404, an oriented film 405 is formed. By applying an electric field via the transparent electrodes 404 to the liquid crystal layer 403, the orientation of the liquid crystal is changed. Because of this change, incident light is reflected by the reflector plate 408, changing the strength of the incident light and producing images.
However, there is a problem in the first reflection type liquid crystal display 400 using the conventional TN system, which is described below. The incident light passes through the two polarization plates 406 and 407 twice one way, that is, four times return trip. Because the two polarization plates 406 and 407 absorb not only polarized light in a direction of the absorption axis but also slightly polarized light in the direction of the transmission axis, the light incurs an optical loss every time the light passes the polarization plates 406 and 407. The optical loss decreases brightness of the display so that the first reflection type liquid crystal display using the two polarization plates 406 and 407 cannot produce bright images.
Between the liquid crystal layer 403 and the reflector plat 408, the transparent lower substrate 402 with a thickness of approximately 1 mm is provided. When a viewer looks at the device obliquely, a doubled image may appear because of parallax. Although one of the reflection type liquid crystal displays uses a super twisted nematic (hereinafter referred to as STN) in which the twist angle of the liquid crystal layer 403 is increased to approximately 200 degrees, the STN device has the disadvantages of decreasing brightness and the problem of parallax, like the TN system, because the STN device uses the two polarization plates.
To solve the problem, a report by A. R. Kmetz, which appears in xe2x80x9cProceedings of the SIDxe2x80x9d Vol. 21, No. 2, pp. 63-65, describes a reflection type liquid crystal display using a single polarization plate. Because the incident light passes through the polarization plate only twice, this reflection type liquid crystal display reduces the optical loss as compared with the first conventional liquid crystal display. Since the polarization plate is eliminated from the rear side of the liquid crystal layer, the electrodes of the substrate facing the light inputting side may be formed of metal, etc., as a reflector electrode, which can solve the problem caused by the parallax.
Japanese Patent Application, First Publication No. 4-116515 discloses a reflection type liquid crystal display using a single polarization plate, and the optimization of three parameters of the twist angle of a liquid crystal layer, the product (hereinafter referred to as xcex94nd) of anisotropy of the refractive index of the liquid crystal layer and its thickness (hereinafter referred to as a cell gap), and the angle of the polarization plate. The angle of the polarization plate is between either the transmission axis or the absorption axis and the direction of the orientation of the liquid crystal in the light inputting side. FIG. 21 is a cross-sectional view showing the structure of the second reflection type liquid crystal display disclosed in Japanese Patent Application, First Publication No. 4-116515. A transparent electrode 404 and a stretched film 405 are formed on an upper substrate 401. A reflector electrode 409 and a stretched film 405 are formed on a lower substrate 402. The liquid crystal layer 403 is provided between the upper and lower substrates 401 and 402, which are arranged in parallel. The light inputting side of the upper substrate 401, that is, the surface opposite to the transparent electrode 404, is coated with a polarization plate 406.
According to the optimized design parameters taught by Japanese Patent Application, First Publication No. 4-116515, the twist angle of the liquid crystal layer 403 is 63 degrees, the And of the liquid crystal layer 403 is 0.2 xcexcm, and the angle of the polarization plate is zero.
In the second conventional liquid crystal display 400 designed according to the above parameters, linearly polarized light output from the liquid crystal layer 403 is converted into circularly polarized light. Further, the light, which is reflected and is transmitted through the liquid crystal layer 403, is converted into a linearly polarized light whose polarization plane is rotated from that of the incident light by approximately 90 degrees. Therefore, the polarization conversion by the liquid crystal layer 403 is remarkably efficient.
Further, a report by S. -T. Wu in xe2x80x9c1997 SID International Symposium Digest of Technical Papersxe2x80x9d Vol. 28, pp. 643-646, 1997 teaches optimized design parameters in the second conventional reflection type liquid crystal display 400 using a single polarization plate. The design parameters described in the report are shown in the following: the twist angle of the liquid crystal layer is 90 degrees, the value xcex94nd of the liquid crystal layer is 0.25 xcexcm, and the angle of the polarization plate is 20 degrees. The report teaches other design parameters for the reflection type liquid crystal display: the twist angle of the liquid crystal layer is 70 degrees, the value xcex94nd of the liquid crystal layer is 0.28 xcexcm, and the angle of the polarization plate is 20 degrees.
The element designed according to these parameters is a normally black element. That is, the element is in a dark state when no voltage is applied, while the element is in a light state when a voltage is applied.
FIG. 22 is a cross-sectional side view showing a third conventional reflection type liquid crystal display. Except that a quarter wavelength plate 410 is inserted between the polarization plate 406 and the upper substrate 401, the third conventional reflection type liquid crystal display 400 is identical to the second conventional liquid crystal display. The third liquid crystal display 400 has a normally white element.
The quarter wavelength plate 410 is a birefringence plate which produces an optical path difference of a quarter wavelength between linearly polarized light oscillating in a vertical direction. The quarter wavelength plate 410 converts the linearly polarized light into circularly polarized light, which is rotated to the right or the left. Thus, the third conventional liquid crystal display is in the light state when no voltage is applied, while the display is in the dark state when a voltage is applied. This normally white element of FIG. 22 reduces dependency on the cell gap and on the wavelength, and is more preferable than the second conventional liquid crystal display.
Japanese Patent Application, First Publication No. 10-232390 discloses a reflection type liquid crystal display using the STN liquid crystal layer and a single polarization plate whose twist angle is above 180 degrees, preferably is 240 degrees. The design parameters of the liquid crystal layer, the phase difference film, and the polarization plate, are optimized so as to make the displayed colors achromatic and to increase the contrast ratio.
Japanese Patent Application, First Publication No. 7-146469 discloses a reflection type liquid crystal display which comprises an upper substrate, a lower substrate, a liquid crystal element with nematic liquid crystal molecules twisted by 90 degrees between the upper substrate and the lower substrate, a polarization plate, a reflector plate, and a quarter wavelength plate. In this background art, an angle between the light absorption axis of the polarization plate and the longitudinal axis of the liquid crystal molecules is 85 to 95 degrees. An angle between the retardation axis of the quarter wavelength plate and the longitudinal axis of the liquid crystal molecules adjacent to the quarter wavelength plate is zero. The product xcex94nd of the anisotropy of the refractive index xcex94n of the liquid crystal layer and the thickness of the liquid crystal layer d is 500 to 700 nm.
Japanese Patent Application, First Publication No. 5-100114 discloses a laminated polarization plate for producing circularly polarized light from monochromatic light, which comprises a half wavelength phase difference film and a quarter wavelength phase difference film.
Japanese Patent Application, First Publication No. 10-090727 discloses a reflection type liquid crystal display in which an absorption axis of a polarization plate forms 35 to 55 degrees, preferably 40 to 50 degrees with respect to an orientation direction of liquid crystal molecules adjacent to a boundary of a substrate. The product of anisotropy of the refractive index of the liquid crystal layer and the thickness of the liquid crystal layer is 0.1 to 0.18 xcexcm, and preferably is 0.12 to 0.16 xcexcm.
Japanese Patent No. 2798073 discloses a reflection type liquid crystal display which comprises a liquid crystal layer and a compensation layer. The anisotropy of the refractive index of the compensation layer is identical to that of the liquid crystal layer. Then, the optical axis of the compensation layer intersects the longitudinal axis of the liquid crystal layer at a right angle, and the angle between the optical axis of the compensation layer and the absorption axis or transmission axis of a polarization plate is 45 degrees or 135 degrees. In the background art, dependency of the amount of birefringence on the wavelength in the liquid crystal layer is cancelled by the compensation layer.
When the conventional reflection type liquid crystal display using the single polarization plate and the quarter wavelength plate allows the circularly polarized light to enter the liquid crystal layer, while working as a normally white element, the birefringence effect may be lost because the liquid crystal molecules rise in the direction of an electric field (substrate normal direction) due to the applied voltage, and the element changes to the dark state.
However, even when a voltage is applied, the liquid crystal molecules strongly fixed adjacent to the boundary of the substrate causes a slight birefringence effect, degrading the contrast ratio. On the other hand, when the twist angle of the liquid crystal layer is 90 degrees as described in the above background art, the remaining birefringence effects at the upper and lower substrates are cancelled, increasing the contrast ratio. Preferably, in consideration of the brightness and the cell gap margin under no voltage, the twist angle is below 80 degrees, and more preferably is 70 degrees, so that the brightness and the contrast ratio are incompatible.
Further, when a viewer looks at the conventional display obliquely, the retardation of the liquid crystal layer and the phase difference film is changed. That is, the display may be yellowish depending on the viewing angle, and this gives a user an unpleasant impression.
The technique disclosed in Japanese Patent Application, First Publication No. 10-232390 can produce achromatic images, whose colors may be changed depending on change in viewing angle, and this gives a user an unpleasant impression.
It is therefore an object of the present invention to provide a reflection type liquid crystal display which can increase the contrast ratio and the brightness of images.
In order to accomplish the above object, a normally-white reflection type liquid crystal display comprising: a polarization plate; a quarter wavelength plate; a transparent substrate, a transparent electrode; and a twisted nematic liquid crystal layer, wherein the polarization plate, the quarter wavelength plate, the transparent substrate, the transparent electrode; and the twisted nematic liquid crystal layer are layered in sequence from a light inputting side, and an angle between the optical axis of the quarter wavelength plate and the transmission axis of the polarization plate is below 65 degrees and above 45 degrees, or below 45 degrees and above 25 degrees.
Preferably, the angle between the optical axis of the quarter wavelength plate and the transmission axis of the polarization plate is 39 to 43 degrees, or 47 to 51 degrees.
The twisted nematic liquid crystal layer is not limited to the liquid crystal layer whose twist angle is 90 degrees, and includes liquid crystal layers whose twist angle is 90 to 50 degrees. The normally white display allows transmission of light when no voltage is applied, and comprises one or more polarization plates.
The advantage of the present invention may be enhanced when the quarter wavelength plate is a wide band quarter wavelength plate with stacked uniaxial phase difference plates.
A twist angle of the liquid crystal layer is 60 to 85 degrees. Preferably, the twist angle of the liquid crystal layer is 68 to 74 degrees.
The product xcex94nd of the anisotropy of the refractive index xcex94n of the liquid crystal layer and the thickness of the liquid crystal layer d is 0.20 to 0.35 xcexcm. Preferably, the product xcex94nd of anisotropy of a refractive index xcex94n of the liquid crystal layer and the thickness of the liquid crystal layer d is 0.24 to 0.31 xcexcm. Under these conditions, the reflection type liquid crystal display can increase the contrast ratio and the brightness of images, and can reduce dependency on the cell gap and on the wavelength.
Preferably, the twist angle of the liquid crystal layer is approximately 70 degrees, and the value xcex94nd is approximately 0.27 xcexcm.
Because the angle between the optical axis of the quarter wavelength plate and the transmission axis of the polarization plate is below 65 degrees and above 45 degrees, or below 45 degrees and above 25 degrees, birefringence at the liquid crystal at the boundary of the substrate, which is caused by incompletely circularly polarized light entering the liquid crystal layer, can be compensated. Therefore, when a voltage is applied (in a light state), the reflectance of the display can be reduced, and the contrast is improved.
Because the product xcex94nd of anisotropy of a refractive index xcex94n of the liquid crystal layer and the thickness of the liquid crystal layer d is 0.24 to 0.31 xcexcm, the contrast is remarkably improved even when brightness and the cell gap margin take precedence over other parameters in the design of displays. Thus, the present invention improves the brightness, the contrast ratio, and the cell gap margin.
In another aspect of the present invention, a reflection type liquid crystal display comprises: a polarization plate; a quarter wavelength plate with a half phase difference film and a quarter phase difference film; an upper substrate; a transparent electrode; a liquid crystal layer; a reflector electrode; and a lower substrate, wherein, the polarization plate, the quarter wavelength plate, the upper substrate, the transparent electrode, the liquid crystal layer, the reflector electrode, and the lower substrate are layered in sequence from a light inputting side, and assuming that the orientation direction of liquid crystal molecules adjacent to the upper substrate is zero degrees, and that a twist direction of the liquid crystal molecules from the upper substrate to the lower substrate is positive, the angle of the polarized light absorption axis of the polarization plate is 5 to 35 degrees, the angle of the optical axis of the half phase difference film is xe2x88x9215 to 15 degrees, and the angle of the optical axis of the quarter phase difference film is xe2x88x9275 to xe2x88x9245 degrees.
Preferably, the twist angle of the liquid crystal layer is 66 to 74 degrees, and the product of anisotropy of the refractive index of the liquid crystal layer and the thickness of the liquid crystal layer is 0.21 to 0.31 xcexcm. The quarter wavelength plate may comprise a plurality of phase difference films.
The half phase difference film and the quarter phase difference film in the quarter wavelength plate are formed of polycarbonate high polymer or polysulphone high polymer. The reflector electrode has irregularities on its surface. The liquid crystal layer includes twisted nematic liquid crystal molecules.
The liquid crystal display includes the polarization plate and the stacked quarter wavelength plate, and the angles of the polarization plate, the half wavelength phase difference film, and the quarter wavelength phase difference film are appropriately adjusted. Therefore, a change in retardation of a liquid crystal layer depending on a viewing angle is cancelled by the change in retardation of the phase difference films depending on the viewing angle. When no voltage is applied, the chroma of images does not exceed 20 within a viewing angle of xc2x160 degrees in all directions. Thus, the present invention prevents an undesirable change in color when a viewer looks at the display obliquely.